1. Field of the Invention
This invention relates to a process for gasification of a graphite.
2. Description of the Prior Art
A process for removal of graphite by combustion with oxygen or air has been conventionally employed for pretreatment process in the reprocessing of spent fuel of graphite-moderated reactor such as high-temperature gas-cooled reactor or spent contaminated moderator or reflector of the graphite-moderated reactor. However, this conventional process involves a problem to be solved in lowering the amount of radioactivity which is liberated and minimization of the amount of radioactive waste, that is, in connection with the fixation or separation of radioactive C-14 in combustion off-gas. That is to say, this conventional process is unfortunately an extreme exothermic reaction and requires the fixation of the carbon dioxide gas containing radio-active C-14 by, for example, lime and, therefore, produces a large amount of waste.
As an alternative to the conventional process the inventors have developed this invention wherein the graphite is gasified without the generation of heat by using carbon dioxide gas as the gasifying agent and the carbon monoxide formed is pyrolyzed to recover the carbon and the carbon dioxide gas is recycled.
Since the rate of reaction of the gasification of the graphite by carbon dioxide gas is very slow, the gasification of the graphite by carbon dioxide gas absolutely requires a catalyst.
Iron group elements such as iron, cobalt and, nickel, alkaline metal such as sodium etc., vanadium and halogen are conventionally used as the gasification catalyst; above all iron, cobalt and nickel are known to be high in activity. The catalytic activities of these gasification catalysts conventionally used are maintained during the reaction at reaction temperatures above 1000.degree. C.; however, they can not be maintained until the termination of the reaction at reaction temperatures below 900.degree. C. and the defect of the gasification catalysts conventionally used that the rate of reaction lowers rapidly after 50-80 percent of the graphite has been gasified can not be eliminated. In certain cases, however, for example, if limitations are encountered with the materials of the reactor, it is required to carry out the gasification of the graphite at temperatures below 900.degree. C. The reason for lowering of the rate of reaction has been continuously studied by a number of investigators. However, it goes without saying that the method for adding the gasification catalyst to the graphite is very significant in the gasification of graphite and that the method for adding the gasification catalyst to the graphite should be selected according to the reaction temperature for gasification of graphite, that is whether it is above 1000.degree. C. or it is below 900.degree. C.
Two methods for adding the catalyst are discussed in the fundamental study. One is mixing the powdered graphite with the catalyst powder and the other is soaking the graphite in an aqueous solution of the salt of the catalyst with non-nitric acid solvent. However, taking into consideration that the reprocessing of fuel requires operations carried out under high radioactivity, the complete treatment of the powder and the powderization of the graphite accompanied with the breakage of coated particles should preferably be avoided.
In order to eliminate the defects of the conventional process as stated above, we have found a process for a gasification of the graphite, particularly improvements in the method for adding a gasification catalyst to the graphite in the gasification of the graphite.